occur over a period of five to ten years, during which is a positive feedback since it contributes to the further
time almost all the summer ice can disappear. There are loss of sea ice. As most of the sea ice around Antarctica
indications that abrupt reductions early in the 21st cen- is already seasonal, this mechanism is only relevant to
tury could result in a largely ice-free Arctic in the sum- the Arctic.
mer as early as 2040
23
. However, most current climate
models do not project that the Arctic will be free of ice in Global significance of sea-ice changes
summer this early in the century, so there is still signifi-
cant uncertainty over this issue. Changes in patterns of sea-ice formation and melting
have widespread influences, including on global climate
Another mechanism for enhanced Arctic sea-ice retreat, and ocean circulation patterns. Ocean circulation is driv-
although not for abrupt changes, is linked to the tran- en partly by gradients in the density of water (known
sition from perennial to seasonal sea ice, which intro- as thermohaline circulation, described in Chapter 2).
duces new regions to seasonal sea-ice cover. Increased Sea water density is determined by heat (“thermo-“) and
seasonal production and loss of sea ice along the Sibe- salinity (“-haline”). Most of the salt from the water that
rian Continental Shelf appears to be one explanation for freezes is added to the water mass below the sea ice. This
an enhanced ocean heat transport into the Arctic Ocean process leads to an increase of salinity in the surface wa-
from the Atlantic Ocean
24
. This enhanced heat transport ter in locations where sea ice forms and to freshwater
Figure 5.10: Aerial photographs of the Arctic sea-ice cover prior to melt (left) and during the summer melt season (right).
Photos: Don Perovich
74 GLOBAL OUTLOOK FOR ICE AND SNOW